FIG. 1 of the attached drawings shows the operation principle of an imaging device. When the imaging plane D of the imaging device moves horizontally, the FOV (Field of View) angle A (2w) and the focal length change at the same time, while the image of an object formed on the imaging plane D through a lens system B of the imaging device, which a focal length C, becomes more clear, which makes the object seen closer. Two methods are commonly used to change the FOV angle A. The first method is to alter the focal length of the imaging device, which is the so-called the optical zooming method that is effected by altering the relative position of a zoom lens that constitutes in part the lens system B. The second one is to change the size of the imaging plane D, namely to change the diagonal length of the imaging plane, which is called the digital zooming process.
The optical zooming principle is that the focal length C is changed by moving inner lenses of the lens system B so as to alter the position of the focal point, while the size of the FOV angle A of the lens system B is also changed accordingly thereby zooming in or zooming out the image of the object. When the position of the focal point is altered, the focal length C varies as well. For example, if the focal point is made to move away from the imaging plane D, the focal length C is lengthened while the FOV angle A becomes smaller. Thus, the image of the object within the view scope on the imaging plane D becomes bigger. On the other hand, digital zooming is realized by using an image processor to separately enlarge the image information caught by a sensing unit located in a zone of a sensing component. In digital zooming, the size of the image of one object on the sensing component (equivalent to the imaging plane D) through the lens system B is not changed. Instead, it is realized by intercepting the imaging factors located in a center portion of the sensing component D by software of the camera, and further by enlarging and interpolating by other software to thereby obtain the zoom-in effect.
Among the various imaging lenses currently available in the market, lenses, like Petzval lens, three-lens-type lens and wide-angle lens are commonly used. A Petzval lens usually consists of two separated lens groups with positive focal lengths, and is characterized by its big aperture and small FOV angle A typical three-lens-type lens normally has three single lenses with positive, negative and positive refractive powers respectively. The FOV angle of a three-lens-type lens is bigger than that of the Petzval lens while the aperture is relatively small. The FOV angle of a wide-angle lens exceeds 60° mostly with a symmetric structure having an aperture stop in the center thereof while the other lenses are symmetrically arranged with respect to the aperture stop.
An imaging lens often adopts three groups of optical zoom lenses with advantages of good image resolution and compact-design feasibility. In prior arts, a three-group optical zoom lens normally contains a negative first lens group, a positive second lens group and a negative third lens group. When the zoom lens varies from a short focal length position to a long focal length position, the aperture stop moves towards the object side together with the second lens group, as it is attached to the second lens group which acts as a system of magnification change.
U.S. Pat. No. 7,072,121 discloses such a kind of optical zoom lens. The zoom lens of the US patent includes a positive first lens group, a negative second lens group and a positive third lens group, which are arranged in sequence from an object side to an image side. The second lens group may move along an optical axis so as to change the magnification thereof. The zoom lens satisfies the condition: 3.7<LT/FW<5.4, where LT represents the distance along the optical axis from the object-side plane of the first lens group to the image plane located at the wide-angle end and FW represents the overall focal length of the zoom lens when it is at the wide-angle end. The image-side plane of the first lens group is concave; the second lens group is a double convex lens; and the object-side plane of the third lens group is concave.
In prior arts, there is also negative-positive-positive three-group optical zoom lens, whose second lens group contains three lenses with an aspheric, first surface. Such a zoom lens only adapts to use in a sensing component with a relatively small size or with a magnification less than three (3), although the image resolution thereof is acceptable. However, if it is used in a sensing component with a relatively big size, the image resolution does not meet the requirement due to a bigger imaging plane. Therefore, the known negative-positive-positive type three-group zoom lens does not perform well when it is used for short-distance photography, and thus it is not usable in photographing an object within 60 mm. Moreover, such prior zoom lens occupies a relatively large amount of space, which makes it impossible to meet the requirement of miniaturization as well.
U.S. Pat. No. 7,075,734 shows a negative-positive-positive type three-group optical zoom lens system providing a magnification of about three (3). A first lens group, a second lens group and a third lens group are arranged in sequence from an object side to an image side. When the zoom lens varies from the wide-angle end to the telephoto end, the first and second lens groups move while the third lens group is fixed so that the distance between the first and second lens groups changes and that of the second and third lens groups increases. The second lens group has four or less than four lenses, among which a diffractive surface is formed on one surface, which is not the one that most approaches the object side, of one of the lenses. The diffractive surface meets the condition: 0.2<C/fW<2.0, wherein C is the effective diameter of the diffractive surface and fW is the focal length of the zoom lens system at the wide-angle end. The diffractive surface is designed to decrease the chromatic aberration, and the flare phenomenon can be reduced as well.
However, preparation of the diffractive surface is complicated and expensive, which not only increases the overall cost of the zoom lens system, but also adversely affects the quality improvement of the produced products. Furthermore, it is more important that such a zoom lens system still cannot overcome the above-mentioned shortcomings of the prior arts. That is, it is not suitable for a sensing component with a relatively big size, and thus it cannot be used to photograph an object located in a distance shorter than 60 mm. Moreover, it also requires a relatively large amount of space to accommodate such a zoom lens system.
Therefore, it is necessary to provide a novel three-group optical zoom lens, which not only meets the requirements of miniaturization and high image resolution, but also can be adopted by a sensing component with a relatively big size to be suitable for short-distance photography.